Reverse osmosis system

ABSTRACT

A reverse osmosis system includes a membrane unit, an energy recovery device, high and low pressure inlet lines, and a concentrate line. The membrane unit has a membrane, an inlet for receiving a feed fluid, a permeate outlet for discharging a permeate fluid and a concentrate outlet for discharging a concentrate fluid. The energy recovering device has a turbine portion, a turbine inlet and a turbine outlet, a pump portion, a pump inlet and a pump outlet, a motor, and a motor control unit for controlling the motor. The low pressure inlet line is connected to the pump inlet for supplying the feed fluid at a low pressure. The high pressure inlet line connects the pump outlet with the inlet for supplying the feed fluid at a high pressure. The concentrate line connects the concentrate outlet with the turbine inlet for supplying the concentrate fluid to the turbine portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to EP Application No. 14186806.7, filedSep. 29, 2014, the contents of which is hereby incorporated herein byreference.

BACKGROUND 1. Field of Invention

The invention relates to a reverse osmosis system having an energyrecovery device, as well as the use of such a system. Furthermore and inparticular the invention relates to an energy recovery device to be usedin such a reverse osmosis system.

2. Background Information

Reverse osmosis systems are used for example for the treatment of watersuch as desalination of seawater or brackish water. In such systems asemipermeable membrane is used that can be passed by the water or thesolvent but not by solutes like dissolved solids, molecules or ions. Forreverse osmosis the membrane is supplied with a pressurized feed fluidfor example seawater. Only the solvent for example the water can passthe membrane and will leave the membrane unit as permeate fluid forexample fresh water. The remaining part of the feed fluid that does notpass through the membrane is discharged from the membrane unit asconcentrate fluid for example brine. The feed fluid has to be suppliedto the membrane with a high pressure to overcome the osmotic pressure.

Thus, reverse osmosis typically is a process where a pressurized feedfluid is required and the concentrate fluid leaving the membrane unitstill has a considerably large residual pressure that enables to recovera part of the pressurizing energy as mechanical energy. In seawaterdesalination, for example, the required pressure of the feed fluid(seawater) may be from 45 bar to 75 bar depending among others on thesalinity and the temperature of the seawater. The pressure in the freshwater (permeate fluid) may be between zero and three bars, the pressurein the brine (concentrate fluid) is typically between 2 and 5 bars lessthan the feed pressure, i.e. 40-73 bar.

To provide for the required high pressure in the feed fluid there is aneed for at least one high pressure pump. To recover mechanical energyfrom the concentrate fluid or the brine, respectively, it is known tohave a separate energy recovery device that can be a pressure exchangeror a combination of a turbine with a pump that is driven by the turbine,pressure exchanger or a combination of a turbine with a pump that isdriven by the turbine, often referred to as turbo-charger.

A reverse osmosis system with energy recovery is known for example fromU.S. Pat. No. 8,691,086. This system comprises a so-called hydraulicenergy management integration system (HEMI) having a turbine portion, apump portion and a motor which is controlled by a controller. The brine(concentrate fluid) leaving the membrane housing is fed to the turbineportion of the HEMI to drive the turbine portion which in turn drivesthe pump portion. The outlet of the pump portion is connected to theinlet of the membrane housing. The inlet of the pump portion isconnected to the outlet of a high-pressure pump supplying the pumpportion of the HEMI with the feed fluid. During the normal operatingprocess of the system the required pressure of the feed fluid at theinlet of the membrane housing is generated by the combined action of thehigh-pressure pump and the pump portion of the HEMI.

SUMMARY

Based on that prior art it is an object of the invention to propose adifferent reverse osmosis system with an energy recovery device. Thesystem shall be simple, in particular from the constructional aspect, aswell as safe and reliable.

The subject matter of the invention satisfying this object ischaracterized by the features of the independent claims.

Thus, according to the invention a reverse osmosis system is proposedcomprising a membrane unit for reverse osmosis and an energy recoverydevice, the membrane unit having a membrane, an inlet for receiving afeed fluid, a permeate outlet for discharging a permeate fluid and aconcentrate outlet for discharging a concentrate fluid, the energyrecovering device having a turbine portion with a turbine rotor, aturbine inlet and a turbine outlet, a pump portion with a pump rotor, apump inlet and a pump outlet, a motor with a motor rotor, and a motorcontrol unit for controlling the motor, wherein the turbine rotor, thepump rotor and the motor rotor are operatively connected by atorque-proof connection, further comprising a low pressure inlet lineconnected to the pump inlet for supplying the feed fluid at a lowpressure to the pump portion, a high pressure inlet line connecting thepump outlet with the inlet of the membrane unit for supplying the feedfluid at a high pressure to the membrane unit and a concentrate lineconnecting the concentrate outlet with the turbine inlet for supplyingthe concentrate fluid to the turbine portion wherein the pump portion isthe sole pump for pressurizing the feed fluid from the low pressure tothe high pressure.

The system according to the invention integrates in a single equipmentboth the pumping function for generating the high pressure in the feedfluid as well as the energy recovery function, i.e. the system does nothave a separate high-pressure pump or any other pressure generatingdevice for pressurizing the feed fluid but uses only the pump portion ofthe energy recovery device to pressurize the feed fluid from a lowpressure to the high pressure required for the reverse osmosis process.Compared to known systems this results in a considerable reduction ofthe complexity of the system. For example, since there is no separatehigh-pressure pump the number of connections may be reduced to four,namely two at the inlet and the outlet of the pump portion and two atthe inlet and the outlet of the turbine portion. A conventional systemwith a separate high-pressure pump needs at least two more connections,namely for the inlet and the outlet of the high-pressure pump.

In addition, the overall constructional effort is considerably reducedwhich enables a very compact embodiment. Besides the absence of aseparate high-pressure pump the pipework effort is reduced as well asthe number of required valves. These reductions also reduce the risk ofleakage and increase the reliability of the system. The capital costs ofthe system are reduced. Concurrently the system in accordance with theinvention offers at least an equivalent if not a better energy balanceor energy efficiency, respectively, as compared to known systems.

From the practical point of view it is preferred when the pump portionis designed as a centrifugal pump.

Preferably, the turbine portion is designed as a Francis turbine or as areverse running centrifugal pump.

It is advantageous, when the pump portion is arranged between the motorand the turbine portion or when the turbine portion is arranged betweenthe motor and the pump portion. Arranging the turbine portion and thepump portion adjacent to each other and the motor on one side of thecombination of the pump portion and the turbine portion renders itpossible to design the pump portion and the turbine portion as aconstructional unit resulting in a very compact design.

According to a preferred embodiment of the system the pump portion isdesigned for rotational speeds of at least 4000 rounds per minute.

Depending upon the desired flow and pressure for the respectiveapplication it may be advantageous when the pump portion is designed forrotational speeds of at least 20000 rounds per minute.

In order to design the system in a very compact manner it is a preferredmeasure that the turbine portion, the pump portion and the motor aredesigned as a constructional unit.

According to a preferred embodiment of the invention the torque-proofconnection between the turbine rotor, the pump rotor and the motor rotoris realized by the feature that the turbine rotor and the pump rotor arearranged on a common shaft that is coupled to the motor rotor. Mostpreferred, the common shaft does not comprise any coupling, nor anygear, nor any clutch between the turbine rotor and the pump rotor. Thecoupling of the common shaft to the motor rotor is designed as aconventional coupling for example a mechanical coupling. However thereare other possibilities to provide for this torque-proof connection, forexample by a magnetic coupling or by a combination of magnetically andmechanically couple the rotors. As an alternative the turbine rotor, thepump rotor and the motor rotor may be arranged on a common shaft.Preferably, the common shaft does not comprise any coupling or clutch.

It is preferred when the turbine portion and the pump portion aredesigned with a common casing, in which the turbine rotor and the pumprotor are arranged directly adjacent to each other. The pump rotor andthe turbine rotor are arranged on a common shaft very close to eachother. In particular, the common shaft does not comprise any coupling orany clutch between the turbine rotor and the pump rotor. Thisarrangement enables a very compact and space saving design.

In order to control the flow of the permeate fluid it is preferred toprovide a first valve arranged and designed to control the flow of thepermeate fluid downstream of the permeate outlet.

In order to control the flow of the concentrate fluid it is preferred toprovide a second valve arranged and designed to control the flow of theconcentrate fluid downstream of the turbine outlet.

In order to minimize the constructional effort of the reverse osmosissystem it may be advantageous when the flow connection between theconcentrate outlet of the membrane unit and the turbine inlet is free ofcontrol valves.

In a preferred embodiment of the reverse osmosis system the pump portionand the turbine portion are horizontally arranged to each other.

As an alternative and depending on the specific application it may beadvantageous when the pump portion and the turbine portion arevertically arranged to each other.

In order to prevent the energy recovery device and the membrane unitfrom degradation or damages it may be advantageous to provide apretreatment unit for purifying the feed fluid arranged upstream of thepump inlet and being in flow communication with the pump inlet. Thepretreatment unit may comprise filters for removing sand, grit or othersolid substances or materials from the feed fluid.

A preferred use of a reverse osmosis system according to the inventionis the desalination of water, in particular seawater.

Another preferred use is the treatment of brackish water.

Still another preferred use is the treatment of any fluid by reverseosmosis.

According to the invention there is also proposed an energy recoverydevice for a reverse osmosis system having a membrane unit with amembrane, an inlet for receiving a feed fluid and a concentrate outletfor discharging a concentrate fluid, said energy recovering devicehaving a turbine portion with a turbine rotor, a turbine inlet and aturbine outlet, a pump portion with a pump rotor, a pump inlet and apump outlet, a motor with a motor rotor, and a motor control unit forcontrolling the motor, wherein the turbine rotor, the pump rotor and themotor rotor are operatively connected by a torque-proof connection, thepump inlet being adapted to receive the feed fluid at a low pressurefrom a low pressure inlet line, the pump outlet being adapted forsupplying the feed fluid at a high pressure to the membrane unit, andthe turbine inlet being adapted for receiving the concentrate fluid fromthe membrane unit and wherein the pump portion is designed forpressurizing the feed fluid from the low pressure to the high pressure.

Thus, the energy recovery device according to the invention is designedand adapted such that the pump portion of the energy recovery device isused as the sole pump for pressurizing the feed fluid from the lowpressure to the high pressure that is required for performing thereverse osmosis. There is no need for any additional high pressure pumpsupporting the action of the pump portion of the energy recovery deviceto generate the high pressure. In addition there is no need for anyother pressure generating device, to pressurize the feed fluid from thelow pressure to the high pressure.

The advantages and the preferred measures as well as the preferredembodiments are the same as already explained in connection with thereverse osmosis system.

In particular it is preferred when the pump portion is designed as acentrifugal pump and when the turbine portion is designed as a Francisturbine or as a reverse running centrifugal pump.

According to a preferred embodiment of the energy recovery system theturbine portion and the pump portion are designed with a common casing,in which the turbine rotor and the pump rotor are arranged directlyadjacent to each other on a common shaft that is coupled to the motorrotor. The common shaft does not comprise any coupling or any clutch orany gear between the turbine rotor and the pump rotor. This arrangementenables a very compact and space saving design.

Further advantageous measures and embodiments of the invention willbecome apparent from the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail hereinafter withreference to the drawings. There are shown in a schematicrepresentation:

FIG. 1 is a schematic view of an embodiment of a reverse osmosis systemaccording to the invention,

FIG. 2 is a more detailed schematic view of the energy recovery systemof the embodiment shown in FIG. 1, and

FIG. 3 is a schematic view of an alternative for the arrangement of thepump portion and the turbine portion of the energy recovery device.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a schematic view of an embodiment of a reverse osmosissystem according to the invention which is designated in its entity withreference numeral 1. The reverse osmosis system 1 comprises a membraneunit 3 having a membrane 31 for performing the reverse osmosis process.The membrane unit 3 has an inlet 32 for receiving a feed fluid, apermeate outlet 33 for discharging a permeate fluid and a concentrateoutlet 34 for discharging a concentrate fluid. In a reverse osmosisprocess the membrane unit 3 is supplied with the feed fluid comprising asolvent, for example water, and solutes like dissolved solids, moleculesor ions. Essentially only the solvent can pass the membrane 31 and willleave the membrane unit 3 as the permeate fluid, for example freshwater. The remaining part of the feed fluid is discharged from themembrane unit as the concentrate fluid. The feed fluid has to besupplied to the membrane 31 with a high pressure being high enough toovercome the osmotic pressure. Therefore the concentrate leaving themembrane unit 3 is typically still under quite a high residual pressurewhich may be up to 95% of the feed pressure, i.e. the high pressureunder which the feed fluid is supplied to the membrane unit 3. Thisresidual pressure of the concentrate fluid enables to recover part ofthe pressurizing energy as mechanical energy. For this purpose thereverse osmosis system 1 comprises an energy recovery system which isdesignated in its entity with reference numeral 2 and which isillustrated in more detail in FIG. 2.

In the following description of the preferred embodiment reference ismade to the important practical application that the reverse osmosissystem 1 is used for the desalination of seawater. In such anapplication the feed fluid supplied to the membrane unit 3 is seawater,the permeate fluid is fresh water and the concentrate fluid is brine.However, the invention is not restricted to the desalination ofseawater, it is also suited for other applications like treatment ofwater in general, purification of water or treatment of brackish water.Furthermore, the reverse osmosis system according to the invention maybe used for all reverse osmosis processes in which the concentrate fluidhas a residual pressure being high enough for the recovery of energy.One example is the treatment or hydrotreatment of hydrocarbons inrefineries.

As already mentioned the reverse osmosis system 1 comprises the energyrecovery device 2. The energy recovery device 2 comprises a turbineportion 21 having a turbine rotor 211, a pump portion 22 having a pumprotor 221, a motor 23 having a motor rotor 231 and a control unit 24 forthe motor 23. The turbine rotor 211, the pump rotor 221 and the motorrotor 231 are operatively connected by a torque-proof connection. In thedescribed preferred embodiment the torque-proof connection is realizedby arranging the pump rotor 221 and the turbine rotor 211 on a commonshaft 26 that is coupled to an motor shaft 25 on which the motor rotor231 is mounted. The common shaft 26 does not comprise any couplings,clutches or gears between the pump rotor 221 and the turbine rotor 211.The coupling of the common shaft 26 and the motor shaft 25 may be aconventional mechanical coupling that is known in the art. Thus, thecommon shaft 25 is driven by the motor rotor 231 such that all the threerotors are coupled rigidly to each other and rotate synchronously.However, the torque-proof connection of the three rotors 211, 221 and231 may be realized in different manners, for example by a magneticcoupling or by other mechanical couplings or by a combination ofmechanical and magnetic couplings. As an alternative the three rotors211, 221, 231, namely the pump rotor 221, the turbine rotor 211 and themotor rotor 231 may be arranged on a single common shaft, i.e. the shaft26 and the motor shaft 25 are designed as a single common shaft.

The combination of the turbine part 21 and the pump part 22 is sometimesreferred to as a turbo-charger. In this embodiment the pump portion 22and the turbine portion 21 are arranged vertically to each other, i.e.with respect to the direction of gravity the pump portion 22 is arrangedon top of the turbine part or, alternatively the turbine part 21 may bearranged on top of the pump part 22. As a further alternative theturbine portion 21 and the pump portion 22 may be arranged horizontally(see FIG. 3).

According to the preferred embodiment of the system 1 the turbineportion 21, the pump portion 22 and the motor 23 are designed as aconstructional unit. By this measure a very compact, space-saving andreliable design of the energy recovery device 2 is realized.

A further preferred measure is designing the turbine portion 21 and thepump portion 22 with a common casing 20 in which both the turbine rotor211 and the pump rotor 221 are arranged adjacent to each other. Thedistance between the turbine rotor 211 and the pump rotor 221 is smallto enable a compact and space saving design of the common housing 20.

The pump portion 22 of the energy recovery device 2 comprises a pumpinlet 222 for supplying the feed fluid i.e. the seawater to the pumpportion 22 and a pump outlet 223 for discharging the pressurized feedfluid from the pump portion 22. Preferably, the pump portion 22 isdesigned as a centrifugal pump. It may be a single stage pump or amultistage pump. Furthermore, the pump portion 22 is designed for veryhigh rotational speeds of at least 4000 rounds per minute (rpm), andmore preferably for at least 20000 rpm. In this embodiment the pumpportion 22 is a high-speed pump or turbo-pump. The high rotational speedof the pump portion 22 enables the creation of a high pressure in thefeed fluid as required by the reverse osmosis process.

Furthermore, the high rotational speed together with arranging the pumprotor 221 and the turbine rotor 211 on the same common shaft 26 resultsin a higher efficiency and thus in a higher amount of energy that can berecovered.

In addition, the high rotational speed renders possible to use smallervanes for the pump rotor 221 as compared to a rotor turning at a lowerspeed and creating the same pressure. This is advantageous in view of acompact design.

The turbine part 21 of the energy recovery unit 2 comprises a turbineinlet 212 for receiving the concentrate fluid, i.e. the brine that isdischarged from the membrane unit 3 and a turbine outlet 213 fordischarging the concentrate fluid from the turbine part 2. The turbinerotor 211 is preferably designed as a centrifugal rotor. Most preferredthe turbine portion 21 is designed as a Francis turbine or as a reverserunning centrifugal pump.

The motor 3, preferably a high speed electric motor, is adapted for thesame high rotational speed as the pump rotor 221 that usually exceedsmultiple times the synchronous speed given by the product of the polepair number and the frequency of the driving current. Preferably themotor control unit 24 comprises a variable-frequency drive (VFD) whichis per se known in the art.

The reverse osmosis system 1 optionally comprises a pretreatment unitfor the pretreatment of the feed fluid, here the seawater. Thepretreatment may be a chemical treatment, a disinfection treatment,separation treatments based on different filtration types like sandfilters, cartridge filters, ultrafiltration and so on or combinations ofsuch treatments. In the case of seawater as the feed fluid thepretreatment unit 4 should remove impurities, solid particles andbiological life which could damages especially the membrane 31. Ofcourse the pretreatment unit 4 may comprise at least one pump (notshown) or other means or device for conveying or moving the feed fluidthrough the pretreatment unit 4.

The components of the embodiment of the reverse osmosis system1 areconnected to each other in the following manner: The seawater as feedfluid is supplied to the pretreatment unit 4 by a supply line 5. Theoutlet of the pretreatment unit 4 is connected to the pump inlet 222 bya low pressure inlet line 6 through which the pretreated feed fluid issupplied to the pump part 22. Thus, the low pressure inlet line 6provides the flow communication between the pretreatment unit 4 and thepump inlet 222. The pump outlet 223 is connected to the inlet 32 of themembrane unit 3 by a high pressure inlet line 7 for supplying the feedfluid to the membrane unit 3. The concentrate outlet 34 of the membraneunit 3 is connected to the turbine inlet 212 by a concentrate line 8 forsupplying the concentrate, i.e. the brine, discharged from the membraneunit 3 to the turbine portion 21. The turbine outlet 213 is connected toa drain line 9 for discharging the concentrate fluid from the turbinepart 21 to a drain 11. In addition, the permeate outlet 33 of themembrane unit 3 is connected to a permeate line 10 for discharging thepermeate, i.e. the fresh water from the membrane unit 3.

In order to control the flows or the flow balance in the system 1 theembodiment of the reverse osmosis system 1 comprises at least twocontrol valves, namely a first valve 12 arranged in the permeate line 10downstream of the permeate outlet 33 to control the flow of thepermeate, i.e. the fresh water and a second valve 13 arranged in thedrain line 9 downstream of the turbine outlet 213 to control the flow ofthe concentrate fluid (brine) to the drain 11. The first and the secondvalve 12 and 13 may be combined each with a flowmeter for themeasurement of the flow through the respective valve 12 or 13 or theremight be separate flowmeters, namely a first flowmeter 14 and a secondflowmeter 15 just upstream of the first and the second valve 12, 13,respectively.

It goes without saying that there may be additional valves or additionalflowmeters to control or to measure the flow at different locations inthe reverse osmosis system 1. Such measures per see are known in the artand do not need any further explanation.

In addition, the reverse osmosis system 1 may comprise several pressuresensors to determine the pressure of the fluid at different locations inthe system. These pressure sensors are not shown in the drawings. Forexample there may be a pressure sensor in the supply line 5 and/or inthe low pressure inlet line 6 and/or in the high pressure inlet line 7and/or in the concentrate line 8 and/or in the drain line 9 and/or inthe permeate line 10 or at other locations. In addition, there may betemperature sensors or other sensors to determine the respectiveproperty of the fluid at different locations in the system 1. Thesignals of the pressure sensors and the other sensors as well as thesignals of the flowmeters 14, 15 are communicated to a control unit thatmay be a separate unit (not shown) or may be integrated in the motorcontrol unit 24. The signals of the different sensors and flowmeters 14,15 are used to determine control parameters which in turn are used toset or to control the rotational speed of the motor 3 as indicated bythe dashed line in FIG. 1—and to control the valves 12 and 13 such thatthe desired flow and pressure in the system is realized.

It is preferred that the flow connection between the concentrate outlet34 of the membrane unit 3 and the turbine inlet 212 is free of controlvalves. Thus, in the described embodiment there is no valve in theconcentrate line 8.

The reverse osmosis system 1 operates in the following manner: Theseawater (feed fluid) is supplied through the supply line 5 to thepretreatment unit 4 where it is pretreated, for example purified byremoving solid particles and biological material. After leaving thepretreatment unit 4 the seawater is at a low pressure with which it isdelivered to the pump inlet 222 of the pump part 22 of the energyrecovery device 2. The term “low pressure” means that the feed fluid isnot considerably pressurized above ambient pressure. Of course, therehas to be a certain pressure to move the feed fluid (seawater) throughthe pretreatment unit 4. Thus, the feed fluid will be at a pressureslightly higher than ambient pressure if there is a pretreatment unit 4.The suction pressure at the pump inlet 222 of the pump portion 22 isusually 2 bars or 3 bars or 4 bars depending on the specificapplication. These pressures are considered as “low pressure” within themeaning of this specification. Especially, the term “low pressure”comprises the pressure values at which the feed fluid is usuallydischarged from the pretreatment unit 4 or the suction pressure at whichthe feed fluid is delivered to the pump providing the high pressure.“Low pressure” may also mean ambient pressure.

The pump part 22 with the pump rotor 221 pressurizes the feed fluid fromthe low pressure to a high pressure being the pressure at which the feedfluid (seawater) is supplied to the membrane unit 3. The term “highpressure” means a pressure that is high enough to overcome the osmoticpressure and to perform the reverse osmosis process in an efficientmanner. The value of the high pressure depends on the specificapplication. For the desalination of seawater the high pressure at whichthe seawater is supplied to the membrane unit 3 depends on severalfactors for example the salinity and the temperature of the seawater.Typical values for the high pressure used in the desalination ofseawater range from 45 bar to 75 bar.

According to the invention the pump portion 22 of the energy recoverydevice 2 is the sole pump for pressurizing the feed fluid from the lowpressure to the high pressure. Different from known reverse osmosissystems using both a high pressure pump and a separate energy recoverydevice the system 1 according to the invention integrates the highpressure pumping function and the energy recovery function in a singleunit, namely the energy recovery system 2. Advantages of this measureare the reduction in the number of connections, the reduction of thepipework effort and the reduction in the number of valves required. Thisresults in a reduced risk of leakage, increased liability and reducedcosts of the system 1.

The pressurized feed fluid (seawater) leaves the pump portion 22 at thepump outlet 223 passes through the high pressure inlet line 7 to theinlet 32 and is supplied to the membrane unit 3 with the high pressure.The fresh water (permeate) leaves the membrane unit 3 through thepermeate outlet 33 and the permeate line 10. For the application ofseawater desalination typical values for the permeate pressure (pressureof the permeate at the permeate outlet) range from 0 bar to 3 bar. Atypical value for the permeate fluid flow is about 45% of the feed fluidflow.

The concentrate, i.e. the brine, is discharged from the membrane unit 3through the concentrate outlet 34 and passes through the concentrateline 8 to the turbine inlet 212 of the turbine portion 21. For seawaterdesalination the brine pressure (pressure of the concentrate at theconcentrate outlet 34) is typically about 95% of the high pressure or 2bar to 5 bar below the high pressure with which the feed fluid is fed tothe membrane unit 3, thus it usually ranges from 40 bar to 73 bar. Atypical value for the concentrate fluid (brine) flow is about 55% of thefeed fluid flow.

The pressurized brine drives the turbine rotor 211 and is dischargedfrom the turbine portion 21 through the turbine outlet 213 and the drainline 9 to the drain 11. By this driving action of the pressurized brineenergy is recovered that is used to drive the pump rotor 221 of the pumpportion 22.

The flow balance in the system 1 is controlled by the first valve 12being arranged in the permeate line 10 and the second valve 13 beingarranged in the drain line 9. These valves 12 and 13 are controlled onthe basis of the flows that are measured by the flowmeters.

The rotational speed of the pump rotor 221 and the turbine rotor 211 isalways set by the motor 23. Depending on the signals delivered by thedifferent sensors (not shown) and/or the flowmeters 14, 15 the controlunit 24 determines the desired rotational speed for the motor 23 andcontrols the motor 23 accordingly.

FIG. 3 shows a schematic view of an alternative for the arrangement ofthe pump portion 22 and the turbine portion 21 of the energy recoverydevice 2. According to this alternative the pump portion 22 and theturbine portion 21 are horizontally arranged to each other. That meansthe pump portion 22 and the turbine portion 21 are arranged side by sidewith respect to the horizontal direction, which is the directionperpendicular to the direction of gravity. All the other explanationsmade in connection with the embodiment illustrated in FIG. 1 and FIG. 2are also valid in the same or in an equivalent manner for thealternative shown in FIG. 3.

The invention claimed is:
 1. A method comprising: providing a reverseosmosis system comprising: a membrane unit for reverse osmosis, anenergy recovery device, a low pressure inlet line, a high pressure inletline, a fluid supply line; a pretreatment unit directly connected to thefluid supply line, the pretreatment unit being a chemical treatmentunit, disinfection treatment unit or a separation treatment unit, andhaving a single outlet directly connected to the low pressure inletline; and a concentrate line, the membrane unit having a membrane, aninlet configured to receive a pretreated feed fluid from the singleoutlet, a permeate outlet configured to discharge a permeate fluid, anda concentrate outlet configured to discharge a concentrate fluid, theenergy recovering device having a turbine portion with a turbine rotor,a turbine inlet, and a turbine outlet, a pump portion with a pump rotor,a pump inlet and a pump outlet, a motor with a motor rotor, and a motorcontrol unit programmed to control the motor, the pump portion being afirst pump in the reverse osmosis system, and the turbine portion, thepump portion and the motor being a constructional unit with the turbineportion arranged vertically with respect to the pump portion, theturbine rotor, the pump rotor and the motor rotor are operativelyconnected by a torque-proof connection, the low pressure inlet linebeing connected to the pump inlet upstream of the inlet of the membraneunit, and carrying the pretreated feed fluid at a low pressure of lessthan 4 bars to the pump portion, the high pressure inlet line connectingthe pump outlet with the inlet of the membrane unit, and supplying thepretreated feed fluid at a high pressure of at least 45 bar to themembrane unit, the concentrate line connecting the concentrate outletwith the turbine inlet, and being configured to supply concentrate fluidto the turbine portion, the pump portion being the sole pump topressurize the pretreated feed fluid from the low pressure to the highpressure; and operating the reverse osmosis system to treat the fluid byreverse osmosis, the fluid optionally being seawater or brackish water.2. A method in accordance with claim 1, wherein the fluid is seawater,and operating the reverse osmosis system includes operating the reverseosmosis system to desalinate the seawater.
 3. A method in accordancewith claim 1, wherein the separation treatment unit is a sand filter, acartridge filter or an ultrafiltration unit.